effect of rearend gearing on ET's

-
mshred

mshred

The Green Manalishi
Joined
May 2, 2008
Messages
3,242
Reaction score
130
Hey guys,

been messing around on desktop dyno and their drag simulator program for awhile and have been a little confused with the results im seeing. Peak HP for my 340 is rated at 417 at 6000 and 402lb-ft torque at 4500. With this combination in a stock geared 4 speed manual, typical track conditions, optimal wheelspin with a 28x 11 inch tire, in an abody mopar (duster) with 4:11 gears im getting a time of 13.168 at 114.6mph launching at 2500 rpm and shifting at 6300 rpm. this is a number im very happy with but heres where the confusion for me comes in...

when screwing around with the rear end gear ratio, I actually noticed that i would get quicker et's by going with a higher gear (lower numerical ratio)...for example, if I change to a 3.23 gear ratio im getting an e/t of about 12.83 at 115.5 mph... This is what im having trouble understanding....I understand that the higher numerical ratios would give me better acceleration, but i thought that would equate to a faster et (especially with the tire size im running) overall as compared to a ratio such as 3.23...My car will be a 95% street driven car, but i want to perform well on the quarter mile as well...I have been told that 3.91 is the highest numerical ratio i should run for a street/strip car without ruining mileage, but from the numbers im getting, im wondering if it would be better to just stick with the 3.23's i have right now...

so i guess what im asking is a couple of things...first, what explains this change in et's as i go from a higher numerical ratio to a lower numerical ratio? What gear will give me more fun on the street (mainly city driving, with some highway on occasion)- the higher ratios like 3.91, 4.10 or the lower ones like 3.55 or 3.23? I dont mind sacrificing some of my et to having quicker acceleration, but if im not going to notice much of a difference from going from 3.23 to something like 3.91 or 4.10 on the street, then maybe i should consider keeping what i have?:dontknow:

feel free to educate me guys...id really like to understand all this:read2:

thanks
Matthew
 
The final drive ratio is higher with the 4.10's in 4th gerar than the 3.23's in 3rd gear. with the 3.23 you would be going through the traps in 3rd rear in sted of 4th with the 4.10. Exeleration would not be as good with the 3.23 set up.
 
Hey Matthew,

A quick observation, your engines peak horsepower is @ 6000 rpm, and your peak torque is @ 4,500 rpm, try shifting @ 5,800 rpm, just to compare the results.

For street and/or fun maybe try a gear somewhere in the 3:73 ratio.
Keep playing with the desktop Dino. It does sound kinda strange that you are getting slower ET's with the 4:11's.

All the best,
Giles
 
hey giles,

its been a while, hope everything is going well for you...thanks for the idea...thats something i didnt think of either....shouldi be trying a different shift point with my combo and the peak numbers that the dyno shot out for me?
 
screwed around with it a little more...if i shift at 5800 theres an increase in my et....it says that the redline for my engine is 6300 rpms, and it seems that the best times im getting after trying a few different shift points is at 6300 rpm...Also, i looked into what burntorange said and yes if I am using the 3.23's it says that i wont even hit 4th gear during my pass...anybody else want to lend a hand? thanks so far guys for the suggestions
 
With mph at 115 that should have you in the11,s.mine runs108 mph which i should be in low 12,s. See,in I ran 12.8 at 108 my mph shows low 12,s this is with some traction lose and a 4 speed.I,m upping my gear from 4.10 to 4.57 and hopefully gonna run high11,s with some tweaking and practise.Your destop e.t,s don,t sound right??115mph in the 13,s ??I,m confured.
 
I'm doin' good,how 'bout yourself and G.?

I'd play with it as much as I could, to find the best result, although high 12's/ low 13's are quite respectable times for a street car. Only change one variable at a time, makes it easier to keep trac of gains and/or losses.

You have at least 4 months of desktop testing ahead of you, make the best of it.

Later,
Giles
 
Hey Giles, me and my dad are doing great...and yes a high 12/ low 13 street car is definatly great for a street car, and definatly fast enough for me if i want to live through the summer...and your definatly right about messing around for the next few months on the desktop- its all i can do now that our good old canadian winter is setting in...

Pettyblu, you are right about the mph sounding kind of off for the timeslip...there could be some factors i might have mushed on, so ill double check, but im pretty sure i got everything pretty close to correct...dont know what this means, but as an example shifting at 6000 rpm with a 3.55 gear puts me at 12.84 at 113.4 mph, where as the same shifting with a 3.23 gear puts me at 12.86 at 115 mph....kinda fishy aint it?
 
Something is seriously wrong. What do you have as the car's weight?

Have you tried shifting after the peak hp?
 
Those desk top dynos just like engine dynos are good as a tool,if you really want to know how fast it is take it to the track...your saying you got a 13.16@114,at that mph you should be going a LOT faster...
 
Take to the track and run it. The dyno things are not going to be a real situation deal. Every car is different.

What's the 60'.... I bet it's over 2 seconds. Does the program provide that?

Running 114 you should be in the 11.60's

Don't kill it leaving the line. Try leaving at 1500-1800 and roll it out. The right way to do it is to slip the clutch until the wheel speed and engine speed match up. It's tough to get a handle on with a bad hooking car and hard on the clutch. The FAST guys have it down, BUT, they usually have some REALLY expensive clutch set ups.

Sounds like you're blowing the tires off it at the hit. I wouldn't bother swapping out gears until you get it to hook with your current set up. If it's mostly a drag, street/strip car, work on the front and rear suspension, weight bias, tire pressures and anything else you can think of to get it to hook.
 
okay well i have the correct gear ratios for the tranny entered, stock duster weight (they let you pick vehicle- only abody there it says 3410lbs), and the rollout is set at 0 ( i dont know if it should be altered?)...my scamp isnt actually driveable yet, but im screwing around with the program to just get an idea how my combination will fare...and yes cracked back my 60 ft time is about 2.7...however, how does that explain my high mph? I tried slowing the launch speed down to about 1500rpm and it only made me a couple of tenths slower and dropped my 60 foot to about 2.5...there is something about a traction coefficient which is right now 1.68...what should it be?
 
mshred said:
okay well i have the correct gear ratios for the tranny entered, stock duster weight (they let you pick vehicle- only abody there it says 3410lbs), and the rollout is set at 0 ( i dont know if it should be altered?)...my scamp isnt actually driveable yet, but im screwing around with the program to just get an idea how my combination will fare...and yes cracked back my 60 ft time is about 2.7...however, how does that explain my high mph? I tried slowing the launch speed down to about 1500rpm and it only made me a couple of tenths slower and dropped my 60 foot to about 2.5...there is something about a traction coefficient which is right now 1.68...what should it be?
Click to expand...

That 60' times is at least 1 second slow from a decent hooking car running that MPH.

Mess with the traction coefficient and see if the 60' time decreases to the 1.60-1.69 range. That's the area it should be for a 114 car running a decent ET.
 
What you need is the Mopar chasise manual. It will tell you all this stuff. Leave the computer games to the kids.
 
crackedback said:
That 60' times is at least 1 second slow from a decent hooking car running that MPH.

Mess with the traction coefficient and see if the 60' time decreases to the 1.60-1.69 range. That's the area it should be for a 114 car running a decent ET.
Click to expand...

Yes i know...ive tried messing with the traction coefficient but i guess i will have to some more....now what mph should it be saying if my car is in the low 13's (which is what my setup should be netting)? Also, does why question of gear ratio out of the question now? Does it make sense with what i originally asked, even though the numbers im getting are totally off?

408demon said:
remember your runnig a four speed
Click to expand...

not to sound rude, but what are you meaning by that? i programmed it as a 4 speed with the 4 ratios of the transmission that i will be using

Burntorange70 said:
What you need is the Mopar chasise manual. It will tell you all this stuff. Leave the computer games to the kids.
Click to expand...

I have read the mopar chassis manual section on manual transmissions...it has nothing to do with this stuff, unless you mean other sections of the manual...and i actually like the computer games if i could just figure out how to work it- having predictions is nice to give an idea of what i will have

thanks so far guys
 
I haven't messed with the drag simulator yet, but something is goofy with this. Does it indicate traction problems and such? If the car is hooked up for the entire pass, I'd think maybe the simulated parking brake was partially engaged for the first 400 feet.

I may snag a trial version for experimentation. These things bug me.
 
If you have the chassis book.* Read page 53 (450 HP A engine case study) andpages 267 and 268.* That should get you close.* Then you can bounce it off of the computer program. Be interesting to see if you get the same results from both.
 
what are you using for the weight of the car? Almost sounds like the weight is way too low.
 
in this thread is where i got the full version- just downloaded it and followed the install notes
http://www.svtperformance.com/forums/showthread.php?p=6474237

As far as vehicle weight goes i have it entered at 3410lbs which they say is stock chassis weight for a duster....either way i agree with you guys, something is seriously wrong...i will read those pages of the chassis manual and see what i can figure out
 
You are using a mathematical formula to simulate something that can't be simulated well. So your results are skewed. It doesnt take into account your clutch (assumes 0 slippage I'm sure), doesn't account for the car's suspension (again, assumes an average reaction), doesnt look at wheel spin past teh initial launch, and doesnt look at the driver, who is the single biggest limiter in most 4sp cars. It's like trying to figure out what a recipe from the internet will taste like by reading the PC screen. You can imagine it, but little else until you make it and try it. MPH indicates true potential in reality. If you run 114 but take 13.1 seconds to reach it, you have a lot of room to improve. That's all you need to knew really. I gear a car for what I do with it. If you drive it daily, don't run 4.56s...lol. If you are racing it, you'll go faster with more gear. A 28" tire should have a roll out of 87-88". Roll out is the length in inches the tire needs to make one full revolution. I have no idea why they ask tire diameter if they also as roll out. At least that's what I recall it being..lol.
 
As far as the projected numbers of the desktop dynoe go, they definatly need some work....im going to do my homework reading the mopar chassis manual (no other homework to do since my school is on strike so mine aswell do ma mopar homework) and then i'll worry about playing around with the desktop dyno later on...I guess my main concern is my selection of gears and the results it was giving me..like i said, as i changed from a higher numerical ratio i.e. 4.10 to lower ones like 3.73, 3.55 i noticed my et was faster...does this have to do with the desktop dyno screwing up or not having correct input numbers, or is this actually true? this was the answer i was originally after and the science behind the answer is what im interested in...Moper, i agree with everything you say, and from what i new, i figured the more gear the better for racing....anyone else want to offer their insight on this topic please do

thanks guys
Matthew
 
i guess my original question never made any sense then? lol
 
It's not simply "more gear is better"... It's the car as a whole. Stock Eliminator cars (and bracket racers) run as much gear as the engine can take, and build the engine to make power at the higher rpms. Because the physics is the same.. X number of horsepower per firing event x number of firing events in a minute = more power delivered to the tires in the same amount of time. It's why Indy cars run to 20,000 rpm, motorcycles run 13000, and Stock Eliminator run sat 6800-8000, nitor dragsters run to 8500+(and why NHRA raised the gear limit a few years ago to slow them down). I use the work the car needs to do, and the driveline package as a whole to determine the gearing. Many times the gearing gets matched to the engine after the engine is done, and it's a common "mismatch". Generally, the most (numerically highest) gear that your engine can turn will get you faster. If the engine falls over at 4000 rpm, and the gearing wants it to spin to 6... you wont go any faster...lol
 
You must log in or register to reply here.
-

Back
Top